1. Field of the Invention
The present invention relates to a diesel engine or the like internal combustion engine having a precombustion chamber and more particularly to improvements in or relating to a ceramic insert for defining a precombustion chamber in a cylinder head of an internal combustion engine as a diesel engine.
2. Description of the Prior Art
A precombustion chamber for a diesel engine or the like internal combustion engine has heretofore been defined by metal parts for the reason of the easy machining and low cost. Recently, it has been practiced to use a ceramic insert or inserts for defining the precombustion chamber with a view to improving the thermal efficiency. durability, particulate emission control, etc.
FIG. 9 shows an example of a prior art insert assembly (A) that includes a domed insert (D) defining a nearly semispherical upper part of a precombustion chamber (I) and a cup-like base insert (F). The domed insert (D) is formed with a glow plug hole (C) and an fuel injection nozzle hole (B). The glow plug hole (C) is inclined with respect to the center axis of the precombustion chamber (I). The glow plug hole (C) is inclined so as to have an inner end (J) that makes an angle .theta..sub.1 with respect to a plane tangential to the precombustion chamber (I).
In another example of a prior art insert assembly (A) shown in FIG. 11, the fuel injection nozzle hole (B) is drilled by a flat-ended drill and is inclined so as to have an inner end (G) that makes an angle .theta..sub.2 with respect to a plane tangential to the precombustion chamber (I).
In the prior art structures, the fuel injection nozzle hole (B) and the glow plug hole (C) are located close to each other and form a sharp angled or edged portion in the domed insert (D). causing a problem that the domed insert (D) that is made of ZrO.sub.2 or Al.sub.2 TiO.sub.5 is likely to chip or break at the sharp edged portion during preparation thereof and furthermore it is likely to be damaged at the sharp edged portion by thermal load applied thereto. Furthermore, since the heat insulating ceramics is generally low in thermal transmission efficiency and therefore a large thermal stress does not occur therein, the domed insert (D) is used as a core during casting of the cylinder head and thereby cast into the cylinder head. For this reason, the domed insert (D) is subjected to compression stresses which are likely to concentrate in the sharp edged portion to cause damages as chipping, cracking, etc. In order to overcome the above disadvantages, it has been tried to chamfer the sharp edged portion of the domed insert (D). However, since the fuel injection nozzle hole (B) and the glow plug hole (C) are inclined with respect to the center axis of the precombustion chamber (I), the chamferring of the sharp edged portion requires a difficult machining work, resulting in a low working efficiency and a high manufacturing cost.